The present invention relates to a device and a method for image pickup, and particularly to a device and a method for image pickup that can perform white balance control according to whether an infrared cut filter is present or not.
Of conventional digital video cameras, a digital video camera is proposed which selectively switches an infrared cut filter to a position on an optical axis of a lens group and a position off the optical axis so that the digital video camera can be used for both picture taking using near-infrared light and picture taking using visible light (see for example Patent Document 1).
Adjustment of a color balance of a camera is referred to as white balance adjustment. A method is proposed which controls an RGB integral ratio to 1:1:1 on the basis of a hypothesis that a spatial average of reflectance of a surface of an object in a scene is close to gray (hereinafter referred to as gray world control). This gray world control is detailed in a document of G. Buchesbaum, “A Spatial Processor Model for Object Color Perception,” J. FranklinInst., 310, 1980.
Further, since many light sources can be regarded as a blackbody, a method is proposed which performs control such that a ratio between respective integral values of an R component, a G component, and a B component of an entire screen is the same as an RGB ratio of blackbody curve data L as shown in FIG. 1 (hereinafter referred to as blackbody curve control). In FIG. 1, an axis of ordinates indicates ratio (B/G) between the integral values of the B component and the G component, and an axis of abscissas indicates ratio (R/G) between the integral values of the R component and the G component. The blackbody curve data L indicates ratios between primary color signals R, G, and B based on a white subject taken under a light source at various color temperatures, and represents a curve such that as one of the ratios is increased, the other ratio is decreased. Also, the color temperature rises as the B/G ratio is increased, and the color temperature falls as the R/G ratio is increased. In the example of FIG. 1, integral value ratios when the color temperature is 7500 K, 5800 K, and 3200 K, respectively, are plotted. Details of the blackbody curve control are disclosed in Japanese Patent Publication No. 2751297.
The gray world control assumes that there is no color deviation within a screen. When there is a color deviation, a white balance is disturbed greatly. On the other hand, the blackbody curve control has an advantage of not readily causing a shift in the white balance even when there is a color deviation. Thus the blackbody curve control is now a mainstream white balance system.
However, at a time of so-called night-shot picture taking, or picture taking in a dark place by irradiation with infrared rays, not only visible light but also an infrared component enters a digital video camera. Hence, blackbody curve control performed using the blackbody curve data L (FIG. 1) for visible light which data is included in a microcomputer changes a color balance of the digital video camera due to effects of the infrared rays. Thus rays of light including the infrared rays redden an entire screen.
Accordingly, in a present situation, the entire screen is purposely made to be a monotone screen for output at a time of night-shot picture taking. FIG. 2 is a flowchart of assistance in explaining a process of selectively switching between a time of normal picture taking and a time of night-shot picture taking and performing white balance control.
In step S1, the digital video camera determines whether the infrared cut filter is disposed on the optical axis of the lens group, that is, whether a mode of picture taking under normal visible light is set according to a user instruction. When the digital video camera determines that the infrared cut filter is disposed on the optical axis of the lens group, the process proceeds to step S2, where the digital video camera performs blackbody curve control so that a ratio between respective integral values of an R component, a G component, and a B component of an entire screen becomes the same as the RGB ratio of the blackbody curve data L shown in FIG. 1.
On the other hand, when the digital video camera determines in step S1 that the infrared cut filter is not disposed on the optical axis of the lens group, that is, that a mode of night-shot picture taking under light including infrared light is set according to a user instruction, the process proceeds to step S3, where the digital video camera performs control to make the entire screen a monotone screen for output.
Thus, the digital video camera controls the white balance of the camera according to picture taking conditions at the time of normal picture taking and the time of night-shot picture taking, for example.
In addition, another method of controlling the white balance of a camera is proposed which suppresses reddening of a screen by correcting a color-difference signal R-Y at a time of low illuminance (see for example Patent Document 2)
[Patent Document 1] Japanese Patent Laid-Open No. 2000-59798
[Patent Document 2] Japanese Patent Laid-Open No. 2003-70009
As described above, at the time of night-shot picture taking, a monotone screen is outputted, and therefore natural color reproduction of the camera cannot be realized.
The technique of Patent Document 2 is effective for a vehicle-mounted camera, a front-door intercom camera or the like that assumes outdoor picture taking, but is not suitable for digital cameras that need to take pictures under various light sources such as fluorescent lamps, incandescent lamps and the like.